1. Field of the Invention
The present invention relates to wireless local area networks (WLANs) and in particular to various techniques that can dynamically adjust parameters relating to interference immunity, thereby improving receiver performance.
2. Discussion of the Related Art
Wireless local area networks (WLANs) are becoming increasingly popular as communication networks. The IEEE 802.11 standards provide guidelines for the operation of devices operating in WLANs. The environment in which these WLANs operate can include both external and internal sources of interfering signals.
For example, FIG. 1 illustrates a pair of WLAN transceivers 110 and. 120 communicating in an RF environment 100. Transceivers include both receiver and transmitter components. In FIG. 1, WLAN transceiver 110 forms part of an access point 160 whereas WLAN transceiver 120 is attached to a laptop computer 150.
Two common external interference sources in RF environment 100, e.g. a microwave oven 130 and a wireless telephone 140, may unpredictably emit noise (shown as dotted lines) in RF environment 100. This external noise can undesirably degrade the performance of both WLAN transceivers 110 and 120.
Interference more closely associated with the WLAN transceivers, called internal interference, can also undesirably degrade performance. For example, laptop computer 150 may transmit predictable clock-related emissions. Specifically, CPU clocks in laptop computer 150 or harmonics of those clocks may have spread-spectrum characteristics. For example, the CDC960 clock generator, manufactured by Texas Instruments Incorporated of Dallas, Tex., generates a 200 MHz clock having spread-spectrum characteristics. In this clock generator, spectral spreading is accomplished by repeatedly sweeping a carrier frequency from a nominal value to a value as much as −0.5% of nominal. Unfortunately, spread spectrum clock interference (SSCI) at approximately the operating frequency of an IEEE-802.11a compliant device could sweep in and out of the passband of the WLAN receiver, thereby appearing to be a pulsing, wideband signal.
Additionally, both WLAN transceivers 120 and 130 can include RF processing circuits. These RF processing circuits can generate predictable spurious signals, called spurs, that vary based on channel, board design, and specific boards and/or components on those boards. Spurs are narrow spectral lines (i.e. tones) that can appear within the WLAN receiver passband. Techniques for processing data with spurs at the packet processing level are discussed in U.S. Patent application Ser. No. 10/664,792, entitled “Spur Mitigation Techniques”, filed on Sept. 16, 2003 by Atheros Communications, Inc., and herein incorporated by reference.
Interfering signals may render WLAN transceivers 110 and 120 unable to receive packets. Even when WLAN transceivers 110 and 120 are able to receive packets, they may generate “false detects”, i.e. erroneously characterizing an interfering signal as a valid data packet. After further analysis, WLAN transceivers 110 and 120 can correct their initial characterization and signal an error condition. Unfortunately, this false triggering decreases throughput because WLAN transceivers 110 and 120 may miss reception of a packet while processing a false detection. Moreover, false triggering can delay transmission while the mediums in WLAN transceivers 110 and 120 are falsely declared busy.
Currently, external interference can be mitigated by changing the physical environment of the WLAN, e.g. by repositioning or turning off microwave oven 130. In contrast, internal interference would typically require adjusting transceivers 110 and 120. One such adjustment is reducing the size of the signal at the ADCs to give more range, thereby reducing the number of false detects. To make this reduction, parameters associated with packet detection can be chosen based on empirical measurements. These parameters, called an interference immunity parameter set, are typically chosen based on a worst-case scenario. Note that this sensitivity reduction need not be dB for dB.
However, choosing an interference immunity parameter set for a worst-case scenario undesirably reduces the overall sensitivity of the receiver. Further, such a setting would not necessarily be optimized for each type of interference. That is, various interference signals have different characteristics that require different mitigation strategies.
Therefore, a need arises for an automated control system and method that enhances interference immunity in a WLAN transceiver.